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The menisci are important shock absorbers in the knee. There are two menisci in the knee – one located on the medial side and the other on the lateral side. Together, each meniscus helps with knee stability and the preservation of joint health.
The medial meniscus, on the inside of the knee, absorbs about 50% of impact to the knee joint – thus, it helps to prevent osteoarthritis, especially in somebody who is bowlegged (varus alignment). The role of the medial meniscus is also important in patients who are missing an anterior cruciate ligament (ACL) or who have previously received an ACL reconstruction. The posterior horn of the medial meniscus serves as a backup to anterior sliding of the knee, which can cause an ACL reconstruction to fail if the meniscus is not present to act as a bumper.
The lateral meniscus absorbs more shock than the medial meniscus. The lateral meniscus has been noted to absorb about 70% of the impact on the lateral compartment in the knee. This is why young, active patients can develop arthritis within a few months after having a partial lateral meniscectomy. In addition to shock absorption, the lateral meniscus also plays a key role in the protection of an ACL reconstruction and provides stability in the presence of a torn ACL. In the absence of an ACL and lateral meniscus – there is a significant amount of rotatory instability – this instability can be exemplified by the pivot-shift test.
A proper medical examination is essential for evaluating a meniscus transplant. One must make sure that the knee is stable, has normal alignment, and has normal cartilage surfaces on both sides of the joint. If any of these are lacking, they should be corrected either before, or during the surgical procedure. The amount of surface area the cartilage protects is also important to assess. We prefer to use high field MRI scans to look at the amount of cartilage remaining to determine if there are any bare areas of cartilage with bone exposed. If only one side of the joint has exposed bone surfaces, then it is an option to resurface this area with a microfracture or a cartilage replacement surgery, such as a fresh osteoarticular allograft. When both sides of the joint have bare bone, it is usually difficult to transplant a meniscus and at that point it is felt that the joint is “too far gone” to be a candidate for a meniscus transplant. Thus, it is very important to follow patients who have had their meniscus taken out to make sure that they do not develop severe osteoarthritis.
The evaluation for a meniscus transplant includes x-rays with sizing markers. One must have a meniscus sized to their knee to make sure it is not too big or too small. Proper sizing for a meniscus transplant is very important to make sure that the patient has it fit in the right spot such that it can preserve joint health. In addition, one should obtain long-leg x-rays to make sure that the alignment is neutral or through the opposite compartment or correctible to a normal alignment.
There are some instances where a meniscus cannot be saved and repaired. In these circumstances, we recommend that patients be closely followed to ensure that osteoarthritis is not developed. Osteoarthritis can develop quickly and one should be aware of the symptoms of osteoarthritis, which include pain or swelling with activities. In young patients, we recommend that they follow-up with x-rays on a regular basis to look for bone spurs (osteophytes) and any potential joint space narrowing, especially on standing AP or Rosenberg views of the knee. In those circumstances, one may need a meniscus transplant in the knee to preserve joint health.
A meniscus transplant is potentially the most technically involved surgery in sports medicine. It is well recognized that there is a long learning curve and one needs to have a very adept surgical team to be able to perform this procedure. Both medial and lateral transplants involve making an incision next to the patellar tendon to be able to make an incision into the joint to be able to slide the meniscus into place. In addition, we strongly recommend the use of bone plugs or a bone trough for meniscus transplants due to the multiple studies, which have demonstrated this to be important in preserving meniscal function. For the medial meniscus, a bone plug is placed both at the front and back of the meniscus and a tunnel is reamed to be able to place it into its normal anatomic attachment site. The rest of the meniscus is then sewn into place both at the back, front and sides of the meniscus to make sure that it is held into correct position. Our European colleagues have performed second look arthroscopies on meniscus transplants, and have noted that the healing process takes 6 weeks postoperatively. Thus, we keep our patients nonweightbearing for the first 6 weeks to maximize healing.
The technique of a lateral meniscus transplant involves making a bony trough and sliding the trough with the meniscal attachments into place in the joint. The trough is necessary instead of 2 bone plugs because the attachment sites of the lateral meniscus are between 12 to 14-mm apart. Thus, it is very difficult to be able to correctly prepare bone tunnels separately for the anterior and posterior attachments of the lateral meniscus. The rest of the technique involving sewing the meniscus into place is very similar to the medial meniscus.
Our center has performed over 300 meniscal transplantations. Our published outcomes are very similar to others in the literature both in North America and Europe. In general, about 80% of patients note good or excellent results for resolution of pain and swelling with activities. In addition, when performed with a revision ACL reconstruction, the majority of knees have a restoration of normal stability and a return to improved function.
Returning to impact activities after meniscus transplant is still controversial. In general, we believe that if one has some chondromalacia, which in effect is early arthritis, of the affected compartment, that one should be careful about returning back to impact activities after a meniscus transplant. This is because we believe that one would normally not return back to impact activities if they had their own meniscus, and relying on a cadaver meniscus to function at a high level to prevent too much joint overload of the affected compartment is probably asking too much of that meniscal graft. Thus, we recommend that patients return back to walking, swimming, cycling and the use of an elliptical machine as their main forms of cross training and activities after a meniscus transplant.
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